Pulse width modulation control (PWM control) has conventionally been applied to a power converter (an inverter) for controlling drive of an alternating-current motor.
Japanese Patent Laying-Open No. 2004-48844 (PTL 1) describes relation between a carrier frequency and volume of noise and power loss in an inverter to which pulse width modulation control is applied. Specifically, as a carrier frequency is lowered, volume of noise originating from the inverter becomes high while switching loss decreases. In contrast, according to the description, as a carrier frequency is higher, volume of noise becomes low while loss in a switching element increases. PTL 1 describes control for setting a high carrier frequency when an engine speed is low and lowering a carrier frequency when an engine speed is high, in order to improve fuel efficiency without giving any uncomfortable feeling to the sense of hearing of a person in a hybrid vehicle.
In addition, Japanese Patent Laying-Open No. 2005-278281 (PTL 2) describes control for lowering a carrier frequency to an audible frequency band when such an obstacle as a pedestrian is sensed, as a vehicle control device for notifying a pedestrian of presence of a vehicle.
Meanwhile, Japanese Patent Laying-Open No. 2007-20320 (PTL 3) and Japanese Patent Laying-Open No. 2008-99475 (PTL 4) exist as techniques for lowering noise caused by PWM control.
PTL 3 describes a PWM inverter device for lowering noise in terms of the sense of hearing without increasing loss. Specifically, according to the description, a carrier frequency determining a frequency of a PWM pulse is caused to periodically or randomly fluctuate only within a prescribed frequency range with any carrier frequency being defined as the center. In addition, PTL 3 describes change in range of fluctuation of this carrier frequency based on a motor current value or a frequency command value.
Japanese Patent Laying-Open No. 2008-99475 (PTL 4) describes discrete and periodic change in carrier frequency over time in order to level noise spectra in a desired frequency band in control of a power conversion device. Then, according to the description, a value for a carrier frequency to be changed is determined such that frequencies of harmonics are not superimposed on each other.